(Feature request) Maloja: Store IDs in the extra field.

[duckfromdiscord]

I sent an email in August about this topic but it might've gone to spam.

Maloja has a field included with scrobbles called extra: https://github.com/krateng/maloja/blob/c6cf28896ca836407a1943cae5542d2b83d009cf/maloja/database/sqldb.py#L211.

My suggestion to fix krateng/maloja#249 once and for all is to create a tag or set of tags in there to store IDs, such as the Spotify ID.

[duckfromdiscord]

Nevermind, I see now that this is not possible since the native API doesn't accept anything here. We will have to go to #198